\input amstex
\def\ve{\varepsilon}

$$1/\infty=0\;\Longleftrightarrow\;(\forall\ve>0)(\exists x_0\in\bold{R})\biggl(x>x_0\;\Longrightarrow\;\Bigl|\frac{1}{x}\Bigr|<\ve\biggr)$$
\bye